With regard to semiconductor devices having a semiconductor element and a circuit substrate electrically connected to each other, higher speed and higher capacity in network apparatuses are in great demand. In order to achieve this, it is necessary to increase the mounting density of the semiconductor device and to increase the input/output pin count as the functions become more sophisticated. Japanese Unexamined Patent Application Publication No. hei 11-330160 discloses an example of a technology for corresponding to a higher input/output pin count. Specifically, in this technology, a network apparatus has a semiconductor element that is joined to a surface of a circuit substrate by means of solder balls arranged therebetween in a grid-like fashion.
A circuit substrate has a coefficient of thermal expansion that is greater than that of a semiconductor element. When the semiconductor element and the circuit substrate are being joined to each other at the melting temperature of the solder balls, the two are joined together with the solder in a state where the circuit substrate thermally expands by a large amount whereas the semiconductor element thermally expands by a small amount. For this reason, the circuit substrate and the semiconductor element become stressed in the cooling process of the solder. Likewise, the solder joining the circuit substrate and the semiconductor element together will also be under stress in the course of the cooling process. However, if the solder balls can withstand the stress or the stress is alleviated by plastic deformation so that the solder balls can maintain their shape, the connection between the circuit substrate and the semiconductor element can be maintained.
On the other hand, with the use of lead-free solder in recent years, the soldering temperature tends to be higher. For this reason, a large displacement may occur between the semiconductor element and the circuit substrate during the soldering step, causing an increase in the amount of stress applied to the solder balls. Because the solder balls are substantially spherical and have a large cross-sectional area, a large amount of stress is applied to the solder balls when such a displacement occurs between the semiconductor element and-the circuit substrate. This is because the amount of stress depends on the product of the cross-sectional area and the displacement. When the solder balls receive such a large amount of stress, it becomes difficult for the solder balls to withstand the stress, thus resulting in cracking or disconnection of the solder balls. This is problematic in leading to lower mounting reliability between the semiconductor element and the circuit substrate.